netdev - Re: [PATCH 2/2] IP_GRE: Linearize skb before csum.

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Date:	Tue, 22 Jan 2013 21:02:53 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	jesse@...ira.com, pshelar@...ira.com, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] IP_GRE: Linearize skb before csum.

On Tue, 2013-01-22 at 23:22 -0500, David Miller wrote:
> From: David Miller <davem@...emloft.net>
> Date: Tue, 22 Jan 2013 20:46:29 -0500 (EST)
> 
> > We can't emit bogus checksums on the wire when this is completely
> > under our control.
> 
> BTW consider a legitimate use case where an application has a shared
> memory page between threads that keeps track of statistic counters,
> and it uses sendfile() over an fd backing that mmap()'d area to
> send the statistics out remotely over TCP.
> 
> That's completely legitimate.
> 
> As is an application constantly overwriting a page in the filesystem
> page cache, and another app doing sendfile() over TCP from that
> area.
> 
> In such a situation everyone of your magic retransmits will have a
> bad checksum too.
> 
> You can't say that this behavior is anything other than broken, and
> I don't want to be responsible for a system that's doing broken
> things like that.
> 
> If your GSO over GRE tunnels stuff fundamentally depends upon this
> magic retransmit fantasy working, then yes we have to disable it
> completely.
> --

But then GSO should be disabled right now, GRE or not.

We have a generic problem.

GSO was a bad idea from the very beginning, if you want
such use case. 



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